398 FXUS64 KHUN 232331 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 531 PM CST Tue Jan 23 2018 .UPDATE... For 00Z TAFS. && .NEAR TERM...(Tonight) Issued at 233 PM CST Tue Jan 23 2018 Visible satellite imagery continues to show a broken-overcast stratocumulus deck associated with a low-level thermal trough axis impacting the northern half of the forecast area this afternoon. The cloud cover has been most widespread north of the TN River, and has been sufficient to have an impact on temperatures, with mid-afternoon readings ranging from the l/m 40s along and north of the AL-TN border to the lower 50s south of the river. Although recent model guidance unanimously agrees that northwest flow will diminish overnight as a surface ridge axis builds eastward from a high over the southern Plains, it is not in very good agreement regarding how quickly the stratocu deck will clear the region. The GFS/ECMWF suggest a more rapid clearing trend as the instigating 850-mb thermal trough axis shifts rapidly east- northeastward into the central Appalachians this evening. However, the NAM and UK Met Office models indicate that clouds could linger across the northeastern half of the CWFA through sunrise tomorrow, if not even a few hours longer. This makes the low temperature forecast tonight quite uncertain, and we have attempted to blend guidance to account for these factors. However, lows across the TN counties and northeast AL could end up being a few degrees warmer than currently forecast if clouds indeed persist through sunrise. .SHORT TERM...(Wednesday through Thursday night) Issued at 233 PM CST Tue Jan 23 2018 An amplifying mid-tropospheric shortwave trough is expected to drop southeastward from western NE into MO overnight, and spread rapidly southeastward across TN/KY tomorrow before reaching the southern Appalachians by 00Z Thursday. Although guidance does suggest that weak elevated vertical motions will develop as this system crosses the region tomorrow morning, moisture in the same layer will be insufficient to support more than an increase in cloud cover. Stronger low-level subsidence in the wake of the departing trough will allow any lingering stratocumulus clouds to dissipate shortly after sunrise, leaving sunny skies during the afternoon, and this should allow temps to warm into the lower 50s for most of the area. Northwest flow aloft will increase in the wake of tomorrow morning's trough, providing mostly clear skies on Wednesday night/Thursday morning. At the surface, a broad ridge will translate northeastward across the TN Valley providing calm winds, and the combination of these factors should allow for strong radiational cooling. Lows will likely fall into the mid 20s for most outlying areas, and (although not explicitly mentioned in the forecast) conditions will be favorable for patchy freezing fog in river valleys and near large bodies of water. Light/variable winds will continue on Thursday, with abundant sunshine anticipated as a mid-level shortwave ridge approaches the region from the southwest, and this should provide even warmer max temps in the mid 50s. A slight warming trend will continue Thursday night, as stronger southerly return flow begins to develop. .LONG TERM...(Friday through Tuesday night) Issued at 233 PM CST Tue Jan 23 2018 During the day on Friday, southwest return flow around the surface high (over the Carolinas and off the eastern seaboard) increases, especially near and west of the I-65. Further east, winds may remain a bit more southeasterly keeping highs a few degrees cooler (52 to 56 degrees). Not expecting much cloud cover, so couldn't rule out a few highs reaching 60 degrees, but think higher temperatures in the west will mainly remain below 60 degrees. Models tighten up the pressure gradient in response to the storm system pushing into Kansas and the western Great Lakes region. Thus kept gusts up to around 20 mph in the forecast in the afternoon. Models remain consistent from last night keeping the best convergence and moisture content west of Alabama through Friday night, maybe into Saturday morning. Thus only have isolated to widely scattered chances of light showers in the forecast after midnight on Friday into the early morning hours on Saturday. There will be deep enough moisture and ample convergence for mostly cloudy conditions to develop around midnight across northern Alabama. Newest guidance looks to be coming into a little better agreement on the evolution of the next storm system moving through southeastern Canada during this period and into the weekend. Models tend to develop stronger convergence and deeper moisture ahead of the longwave trough axis associated with this system Saturday afternoon into Saturday night. A complicating factor is that models continue to show some upper level forcing/energy pushing northeast from the central Gulf of Mexico at the same time. However, they are very different with the speed and movement of this feature. This may help to either increase rain chances or keep them lower (ECWMF solution) or delay high precipitation chances (GFS solution). At this point went a bit below blended guidance given the uncertainty with 30 to 40 percent pop Saturday afternoon. Highs in the lower 60s look reasonable given the 925 mb temperatures and mixing of boundary layer. There is quite a spread in model precipitation guidance given the differing solutions with the energy pushing northeast or east- northeast from the Gulf of Mexico Saturday night through Sunday. Thus keeping a bit below guidance, with high chance or likely pops Saturday night into Sunday. Timing of heavier rainfall and if it will hang around through Sunday is very uncertain. Due to limited instability and the uncertainty with this forecast, leaving thunderstorms out of the forecast with through Sunday. Somewhat cooler air returns Sunday night into early next week, as the storm system pushes east of the area. Highs return to the mid 40s to lower 50s and lows drop back into the upper 20s to 30s. && .AVIATION...(For the 00Z TAFS through 00Z Wednesday evening) Issued at 531 PM CST Tue Jan 23 2018 VFR conditions will prevail through the next 24 hours at the KMSL and KHSV terminals. Mostly clear to partly cloudy skies will persist and winds will generally remain from the north-northwest through the period. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...70/DD SHORT TERM...70/DD LONG TERM...KTW AVIATION...73 For more information please visit our website at weather.gov/huntsville.